1. Field of the Invention
This invention relates to radio ATM transceivers which receive and transmit ATM signals via radio lines in the air by means of ATM interfaces (where "ATM" is an abbreviation for "Asynchronous Transfer Mode"). Particularly, this invention provides a radio ATM transceiver which has a capability in ATM-supported multimedia services. This invention is based on patent application No. Hei 8-345891 filed in Japan, the content of which is incorporated herein by reference.
2. Prior Art
FIG. 5 shows a typical architecture of a radio ATM transceiver system which is conventionally known. Specifically, FIG. 5 shows an example of a radio LAN (an abbreviation for "Local Area Network") which provides a radio base station 100 and a number of radio terminals 201 to 20N.
The radio terminal 201 performs handshaking of data packets with a server 100 or other radio terminals 202 to 20N by means of the radio base station 100 which has an interface with a wired LAN 400. Such a data transmission using the conventional radio LAN is, like the Ethernet, a so-called best-effort type transmission which cannot perform transmission of data immediately. For this reason, the above data transmission is not suited to transmission of information such as image signals which require real-time performance.
Both of the radio LAN and Ethernet are based on the MAC collison control method (where "MAC" is an abbreviation for "Media Access Control"). For this reason, there is a possibility that packets from terminals cannot be transported within a certain time when as the terminals issue transmission requests simultaneously. So, the conventional radio data communications method is limited to handling data of non-real-time performance such as characters and image files.
Meanwhile, the ATM system has been introduced as the key technology to realize the B-ISDN (an abbreviation of "Broadband Integrated Services Digital Network"). This ATM system is capable of handling so-called multimedia information such as voice, data and pictures (e.g., image signals) efficiently. Because, in the ATM system, the network guarantees the quality of service which the terminal requires in response to a request of service from the terminal.
Specifically, the ATM system is of the connection-oriented type. So, when starting transmission, the terminal requests a certain quality of service against the ATM switch by a signaling mode. Then, the ATM switch communicates with another ATM switch within the communication network to confirm whether to secure a bandwidth necessary to accomplish the requested service. If it is confirmed that the bandwidth can be secured, the ATM system establishes the connection to provide the service for the terminal. Thanks to the above operation, the ATM system is capable of providing a variety of services such as the following CBR (an abbreviation of "Constant Bit Rate"), VBR (an abbreviation of "Variable Bit Rate") and ABR (an abbreviation of "Available Bit Rate").
In case of transmission of image signals which requires real-time performance, the ATM system supports transmission by CBR or VBR. In contrast, in case of transmission of character data which does not require real-time performance, the ATM system supports transmission by ABR. Incidentally, the ABR is the best-effort type service which performs transportation of data only when vacancy exists in the resource within the network.
To enable communications of multimedia signals such as the image signals even in the areas of radio communications by using the aforementioned advantages of the ATM system, the ATM forum and ETSI (an abbreviation of "European Telecommunications Standards Institute") recently began setting standards for the radio ATM systems.
The conventional radio ATM transceiver has a relatively slow radio transmission speed which is smaller than 155 Mbps for ATM by one digit. For this reason, the conventional technology should perform transmission by assembling multiple ATM cells. As a result, the ATM cells are in burst conditions. So, the conventional technology suffers from the following problems at the side of the clients.
That is, there is a probability to cause deterioration of the quality of service due to the fluctuations of intervals of the cells. In addition, there is a probability to cause the lack of the processing ability due to the reduction of intervals of the cells.
Next, the above problems will be explained in a concrete manner with reference to FIGS. 2A to 2E and FIG. 3.
FIG. 3 shows an example of a system which establishes connections between a VOD server 10 and a VOD client 20 by means of an ATM switch (abbreviated by "ATMSW") 30, where "VOD" is an abbreviation for "Video On Demand". A connection is established between the VOD client 20 and ATM switch 30 via a radio line (or radio transmission path) in the air between radio modems 40 and 50 (where "modem" is an abbreviation for "modulator-demodulator"). Using the above connection, the system performs data transmission. Herein, ATM cells are transported, using the CBR which is normally used for the VOD service, with a transmission speed V (bps) which is required to provide the VOD service.
In the ATM system, the ATM cell has a basic unit whose length is short, i.e., 53 bytes. The radio modem 40 receives ATM cells from the VOD server 10 via the ATM switch 30, so that the radio modem 40 transmits the ATM cells to the radio modem 50 one-by-one in a transparent manner see signal A in FIG. 2A. Due to the transmission capability of the radio modem and the overhead corresponding to an addition of the radio header, there is a probability that the transmission speed V cannot be guaranteed.
To avoid the above problems, the system uses a form of a signal B shown in FIG. 2B, for example. That is, the system transmits multiple ATM cells assembled together. However, when the ATM cells in the signal B are directly passed to the VOD client 20, the ATM cells should be arranged like a signal E of FIG. 2E which corresponds to the VBR of the burst type. In that case, it is not possible to meet the condition of the CBR.
As a result, the conventional system has a probability to cause the deterioration of the quality of service due to the fluctuations of the intervals of the cells as well as a probability to cause the lack of the processing capability due to the reduction of the intervals of the cells. SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a radio ATM transceiver which is capable of providing the multimedia service supported by the ATM using transmission paths corresponding to radio lines in the air.
It is another object of the invention to avoid occurrence of the burst-type ATM cells in the CBR service.
It is a further object of the invention to avoid deterioration of the quality of service due to the fluctuations of the intervals of the cells as well as lack of the processing capability due to the reduction of the intervals of the cells.
A radio ATM transceiver of this invention operates in connection with a transmitting mode and a receiving mode independently. In the transmitting mode, the radio ATM transceiver receives SONET frame signals via an optical fiber, for example. Herein, the SONET frame signals are disassembled into ATM cells, from which signaling information is extracted and is subjected to analysis to recognize a service class (e.g., a CBR service) which is requested for ATM communication. In accordance with the service class, a number of ATM cells are assembled together with an addition of a radio header to provide a radio packet. Then, radio packets are forwarded to a radio modem, from which they are transmitted into the air on radio lines using a prescribed transmission speed. It the receiving mode, the radio ATM transceiver disassembles the radio packet transmitted thereto into a number of ATM cells with excluding the radio header. The ATM cells are converted to SONET frame signals, which are then outputted using a transmission speed which corresponds to the service class.